One type of a multi-stage switch is a three stage switch. Three stage switches have long been used in the telephone industry. Use of three stage switching reduces the number of crosspoints required to construct large numbers of inputs to large numbers of outputs. One embodiment of a three stage switch requires setting up a path from an specific incoming line to an outgoing line or group. In general, multi-stage switches offer alternatives as to which output an input is connected.
Typically, in three stage switches, first stage inputs and third stage outputs are partitioned into subgroups of a given number of crosspoint switches. All center stages in three stage switches provide connections from any first stage to any third stage. In general, the multiple stage structure provides alternative paths through the multi-stage switch from the input to the output.
Blocking of switch connection is said to occur when no open path is available between the first stage and the third stage. It has been shown that if the center stages has overcapacity the switch can be nonblocking. Nonblocking can be derived by first observing that a connection through the center stage requires a switch that has an idle connection to the first stage and an idle connection to the third stage that creates the desired connection. A switch is strictly nonblocking if a desired path from the first stage is always available to the third stage.
Most voice telephone networks accept a certain level of blocking. Accordingly, analysis have been made to evaluate the blocking of telephone three stage switches at given operation levels. Switch capacity can be added to provide excess switch paths to achieve acceptable levels of blocking.
Three stage switches also have been used for video signals. Two major problems affect video three stage switches: blocking and crosstalk. Crosstalk is generally understood to be the interference, or distortion of one information signal caused by another information signal. Crosstalk can be caused by the physical proximity or the strength of signals. Crosstalk and blocking can be easily overcome when space is not a constraint. Blocking can be avoided by increasing the number of switches available so that a path is always available between the input and output. Crosstalk can be avoided in a video switch by increasing the distance between video signals thereby reducing the effect of one signal on another. In the video field, three stage switches tend to be relatively large and heavy because physical space is usually not a constraint.
Such large three stage switches, however, present design problems in applications where space is not readily available such as for use in aircraft and other passenger vehicles. What is therefore needed is a multi-stage switch that reduces or avoids blocking and reduces the crosstalk between video signals while avoiding the size and weight added by the overcapacity of the prior art.